Ion implantation is a technique for introducing conductivity-altering impurities into semiconductor workpieces. During ion implantation, a desired impurity material is ionized in an ion source chamber, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is focused and directed toward the surface of a workpiece positioned in a process chamber. The energetic ions in the ion beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the material to form a region of desired conductivity.
Two concerns of the solar cell manufacturing industry are manufacturing throughput and solar cell efficiency. Solar cell efficiency is a measure of the amount of solar energy that a solar cell is able to convert into electricity, and is closely tied to the precision with which a solar cell is manufactured. As technologies advance, higher solar cell efficiencies may be needed to stay competitive in the solar cell manufacturing industry. Improving precision while maintaining or improving manufacturing throughput is therefore highly desirable.
Ion implantation has been demonstrated as a viable method to dope solar cells in a precise manner. Use of ion implantation removes process steps needed for existing technology, such as diffusion furnaces. For example, a laser edge isolation step may be removed if ion implantation is used instead of furnace diffusion because ion implantation will only dope the desired surface. Besides removal of process steps, higher cell efficiencies have been demonstrated using ion implantation. Ion implantation also offers the ability to perform a blanket implant of an entire surface of a solar cell or a selective (or patterned) implant of only part of the solar cell. Selective implantation at high throughputs using ion implantation avoids the costly and time-consuming lithography or patterning steps used for furnace diffusion. Selective implantation also enables new solar cell designs.
In some cases, micron-level precision may be required for the implantation of certain types of solar cells to achieve necessary geometries and tolerances. For example, selective emitter (SE) and interdigitated backside contact (IBC) solar cells have doped regions that are only a few μm apart. If a mask is used create such doped regions in a workpiece during ion implantation, the locations of the regions are dictated by the placement of the mask and the dimensions and/or geometry of the mask. The mask may therefore need to be repeatedly placed within approximately 20-40 μm of a desired location to reliably meet manufacturing specifications. Otherwise, workpieces may not function as desired or subsequent processes employed in the manufacture of workpieces may not be properly aligned.
Any improvements to the precision, reliability, and speed with which solar cells are manufactured would be beneficial to solar cell manufacturers worldwide and may accelerate the adoption of solar cells as an alternative energy source.